There has been proposed a variety of such apparatus comprising a hydraulic motor and a reduction gear unit for transmitting power to an endless belt with a reduced speed and a high torque, wherein the hydraulic motor and the reduction gear unit usually projected beyond the width of the endless belt. This frequently resulted in damage thereto when they collided with hard obstacles upon running of the vehicle body. Moreover, it is troublesome and laborious to assemble and disassemble mechanical parts of the conventional apparatus.